We English speakers use past participles in many ways:
•Building a perfect verb: I have eaten breakfast already.
•Building a perfect continuous verb: I have been eating for two minutes.
•To describe something: The locked door
•To make a passive construction: The pizza was eaten by my dog!
And perhaps there are even some other uses [^1]. But seeing these uses often just confuses stuents more. So what the heck is a past participle? And why does English have them?
To understand what a Past Participle (PP) is, it is important to understand a phenomenon about English, and also about the brain. That is, that specific categories of words that we know, are not always so, or that concepts cannot always fit perfectly into just one category. For example, let's look at nouns and verbs on a continuum:
Students often think that nouns and verbs are very different, but they're not. Take these two sentences for example:
1) Swimming is fun.
2) Sally is swimming quickly.
In the first sentence swimming is a noun. How do we know that? Well, it is in the subject position, does not change ( Compare with the incorrect "Swimmings are fun."), and must agree with the verb. This is enough evidence to suggest that swimming is not a verb, but a noun in that sentence. Now take a look at the second sentence. Swimming is a verb here. How do we know? Well, for the opposite reasons as the the noun.
But conceptually, when you think of swimming, is there a difference between the noun and the verb? Not really. This idea is somewhat between the two categories, which is why a gerund (noun) is used. But often gerunds get confused for infinitives (verbs), and now you know why.
So what I'm suggesting is that nouns and verbs are really the same things in your mind, but just show up differently in the grammar, or that they are different versions of the same thought. The same is true for Verbs and Adjectives. The participle is halfway between an action and a description, the way a gerund or infinitive is halfway between a noun and a verb. This makes sense. After all, can't we describe something by its movement? For example:
a) Which guy is your brother?
b) The running guy. (Or, the guy who is running.)
a) Oh, I see him.
So the participle allows English speakers to use another method of description, the verbiness--or action--to describe something. It gives us a deeper language with more dimensions.
But don't forget that English has two participles, the past and the the present participle, which is marked by the ing suffix. Compare: 3) The boring professor 4) The bored professor
English allows for an active (present participle) and passive (past participle) descriptions. In other words, the description can be for causing or receiving the description.
[^1]: Email me if I missed something.